1. Field of the Invention
The present invention relates to high temperature fuel cell generators, wherein depleted fuel and depleted air are kept separate from each other to allow treatment of depleted fuel to generate and capture essentially pure carbon dioxide, thereby precluding the release of greenhouse gas to the environment.
2. Background Information
Tubular solid oxide electrolyte fuel cell (SOFC) generators have been well known in the art for almost twenty years, and taught, for example, by A. O. Isenberg in U.S. Pat. No. 4,395,468. There, in the main embodiment, oxygen (as present in air), as oxidant, was reacted at the inside xe2x80x9cairxe2x80x9d electrode of a closed tubular SOFC, to yield depleted air; and fuel, such as CO and H2, was reacted at an outside xe2x80x9cfuelxe2x80x9d electrode of the closed tubular SOFC to yield depleted fuel, all in a xe2x80x9cgenerating chamber,xe2x80x9d at high temperatures (that is, about 1000xc2x0 C.). The air electrode generally comprised a doped lanthanum manganite, the fuel electrode generally comprised a nickel cermet and an electrolyte disposed between the electrodes generally comprised a stabilized zirconia. The depleted air and depleted fuel were subsequently completely combusted in a separate, but attached preheating chamber, to preheat feed air. This basic SOFC generator design was carried forward, with other improvements, as shown for example in U.S. Pat. Nos. 4,664,986; 5,573,867; and 5,733,675 (Draper et al.; Zafred et al.; and Dederer et al.).
Other designs have used a series of fuel cell stacks, each providing a stage containing a different electrolyte operating at a lower temperature to improve fuel gas utilization, as taught in U.S. Pat. No. 5,712,055 (Khandkar). In a somewhat similar fashion, in one embodiment of U.S. Pat. No. 5,134,043 (Nakagawa), xe2x80x9cdepleted fuelxe2x80x9d from a molten carbonate fuel cell system is sent to a separate molten carbonate anode, where the product was then mixed/contacted with oxidant/air before being introduced into the cathode section of the first molten carbonate electrolyte fuel cell. While tubular fuel cells are emphasized herein, flat or planar fuel cells, which are well known in the art, may also be used.
However, such designs could release byproducts of combustion, such as carbon dioxide into the atmosphere. Efforts are now being made on an international level to globally reduce the release of so-called xe2x80x9cgreen house gasesxe2x80x9d which includes carbon dioxide, which may contribute to global atmospheric warming. Such efforts may, indeed, lead to future legislation regarding carbon dioxide emissions from SOFCs. What is needed is a means to further treat the spent fuel from fuel cell generators to not only reduce or eliminate carbon dioxide emissions, but also to increase the capacity of the fuel cell generators to further utilize feed fuel, thereby producing more electricity. Such a need applies to both tubular and flat plate type fuel cells.
In the area of reducing carbon dioxide emissions from power plants utilizing a variety of types of fuel cells, in order to reduce the xe2x80x9cgreen house effectxe2x80x9d, U.S. Pat. No. 4,751,151 (Healy et al.) taught a carbon dioxide absorber, such as monoethanolamine, including a regenerable absorbent, for stripping carbon dioxide followed by subsequent cooling and compression. In U.S. Pat. No. 5,064,733 (Krist et al.), recognizing prior art conversion of natural gas into carbon dioxide and water-with the accompanying creation of a DC electrical current-in a solid oxide fuel cell, taught conversion of the carbon dioxide and water to C2H4, C2H6 and C2H2 by use of a copper, copper alloy or perovskite cathode. That cathode was in contact with the CO2, and H2O and a dual layered anode made of metallic oxide perovskite next to the electrode with an outer contacting layer of rare earth metallic oxide contacting CH4. This provided for concurrent gas phase electrocatalytic oxidative dimerization of methane at an anode on one side of a solid electrolyte and reduction of carbon dioxide to gaseous hydrocarbons at a cathode on the other side of the solid electrolyte.
Other CO2 treatments include U.S. Pat. No. 5,928,806 (Olah et al.), where a regenerative fuel cell system containing two electrochemical cells in fluid communication were taught, one cell oxidizing an oxygenated hydrocarbon, such as methyl alcohol, formic acid, etc., to CO2 and H2O and a second cell reducing CO2 and H2O to an oxygenated hydrocarbon. This produced methyl alcohol and related oxygenates directly from CO2. Also, U.S. Pat. No. 5,866,090, (Nakagaua et al.) taught treating carbon dioxide effluent, from an energy plant which uses fuel cells, with lithium zirconia at over 450xc2x0 C., to produce lithium carbonate and zirconia.
In the area of separation of gas streams in an apparatus, U.S. Pat. No. 4,801,369 (Draper et al.) taught a solid oxide water electrolyzer using solid oxide fuel cells having oxidant and fuel feeds, where water (in the form of steam) was dissociated to H2 and O2. There, in order to prevent recombination of the H2 and O2 and to eliminate the possibility of fire or explosion, a controlled leakage of additional steam into the H2 and O2 streams, at a high pressure, was used as a separation xe2x80x9csealxe2x80x9d. This separation means allowed a seal-less design, which was important at the 800xc2x0 C.-1100xc2x0 C. electrolyzer operating temperatures. In the invention of Draper et al., a separate steam plenum was used to separate the oxygen and hydrogen collecting means. with this design, however, it was difficult to achieve uniformity of steam leakage, so that there was the possibility of diffusion of O2 and N2 into the steam plenum and subsequent mixing of these gases with the fuel.
While a great many methods to treat carbon dioxide are known, a new fuel cell generator design is needed to allow segregation of the carbon dioxide for such treatment.
Therefore it is a main object of this invention to yield an improved fuel cell generator design, allowing segregation of carbon dioxide generated at the fuel electrodes.
It is a further object of this invention to segregate the depleted oxidant stream from the depleted fuel stream, so that the carbon dioxide can be segregated.
These and other objects are accomplished by providing a high temperature fuel cell generator comprising a generator chamber containing solid oxide electrolyte fuel cells which operate on oxidant and fuel to yield a depleted oxidant stream and a depleted fuel stream consisting essentially of unreacted fuel, CO2 and H2O, where the oxidant and fuel streams do not communicate directly with each other, so that depleted oxidant and depleted fuel remain effectively separated, and where the depleted oxidant stream is separated from the depleted fuel stream by a flow of steam. The depleted fuel exits as a gas consisting essentially of carbon dioxide and water for further treatment, where at least one exit is provided for depleted oxidant to exhaust to the environment. In no instance will the depleted fuel stream contain only H2. The depleted fuel steam will always contain substantial amounts (greater than about 80 vol. %) of CO2 and H2O. The steam separation is preferably effected by means of a separate barrier steam plenum, where the steam is at a pressure higher by approximately 1 psi (6.89xc3x97103 Pa or 6.89 kPa) than either the depleted oxidant stream or the depleted fuel stream, and there may be a controlled leakage of steam into those streams in a seal-less generator design. This higher pressure is achieved by placing a constricting means as a pressure control means in the exit of the barrier steam plenum. Such a constricting means could be selected from an orifice, venturi or the like.
The invention also covers a method of operating a high temperature fuel cell generator containing solid oxide fuel cells with a solid electrolyte disposed between an air electrode and a fuel electrode which operate on oxidant and fuel gases, comprising the steps: (1) feeding feed fuel gas to contact fuel electrodes of fuel cells to yield a depleted. fuel gas stream consisting essentially of unreacted. fuel, CO2 and H2O, and (2) feeding oxidant gas to contact air electrodes of fuel cells to yield a depleted oxidant gas stream; where depleted oxidant gases are kept separated from all depleted fuel gases by a flow of steam. In the operation of this generator, the depleted fuel may be further reacted to yield a gas consisting essentially of carbon dioxide and water.
Thus, this invention relates to an arrangement of components within an fuel cell generator by means of which the exhaust products, such as CO2 and H2O, are segregated from exhausted vitiated air. By this means, water can be condensed from the product exhaust stream, and the carbon dioxide can be pressurized or liquefied and put to use, rather than being released to the atmosphere. Consequently the release of a xe2x80x9cgreen house gasxe2x80x9d pollutant is avoided. The main idea is to produce electric power and sequester carbon dioxide. The enclosure surrounding the generator also contains a means to yield steam to a barrier plenum separating a depleted fuel plenum from a depleted oxidant plenum, as well as stack fuel reformer passages positioned between the fuel cells in the generating chamber.
A main part of the invention is a fuel cell generator where a barrier plenum is interposed between a spent fuel plenum and vitiated air plena, where the barrier plenum is supplied with steam from an external source for the purpose of preventing the migration of depleted fuel into vitiated air (or vitiated air into depleted fuel), thus insuring that combustion does not occur.